It is well known to prepare slow release tablets utilizing an algin gel. Typically, a water soluble alginate such as sodium alginate and calcium ions in the form of a calcium salt are reacted to gelatinize the algin by converting it into an insoluble calcium alginate gel. On the addition of a strong acid to the mixture of sodium alginate and calcium salt, calcium salt is slowly ionized to yield calcium ions. The calcium ions then react with the soluble alginate to form an insoluble calcium alginate. Gelation proceeds through gradual ionization of the calcium salt. With these formulations, the controlled release properties of the alginate gel have been varied by varying the molecular weight of alginate, the alginate concentration, the type of polyvalent cation cross-linking agent or the concentration of the cation.
Exemplary of the prior art is Great Britain Pat. No. 1,355,985 which discloses a solid slow release preparation for prolonged drug action, the preparation containing a mixture of sodium alginate and a slightly soluble calcium salt which, when in contact with gastric juices, liberates calcium ions. The calcium ions react with sodium alginate to form a sponge-like gel of calcium alginate through which a drug slowly diffuses. The gel is formed by calcium ions cross-linking with a water soluble alginate and has a lattice mesh through which water is absorbed. Inert, insoluble ingredients may be added to control the porosity of the lattice mesh. The lattice retards the apparent dissolution of the active ingredient.
The sodium alginate formulations with calcium ions known to date have been unsuccessful in providing the controllable and reproducible release of drugs presumably due top the unpredictable nature of the reactivity of the calcium ions resulting in poorly formed gels and due to the periodic precipitation of insoluble materials into the gastrointestinal tract.
Other references which may be pertinent to the present invention include U.S. Pat. No. 3,640,741 which discloses a plastic mass containing a gel for the slow release of pharmaceuticals. The gel is made by mixing calcium chloride and sodium alginate in a glycol solution containing a pharmaceutical. Similarly Japanese Pat. Nos. 76415 and 76413 disclose the use of sodium alginate in solid pharmaceutical formulations. Lastly, numerous references including A. E. Abotaleb et al., Pharmazie, 38,473 (1983)); E. Shotton et al., J. Pharm. Sci. 65, 1170 (1976); and K. A. Khan et al., J. Pharm. Sci. 64, 166 (1975), all disclose the use of sodium calcium alginate, which is a 50:50 physical mixture of sodium alginate and calcium alginate, in pharmaceutical tablets as disintegrating agents.
In spite of the above-noted prior art, no known pharmaceutical tablet formulation to date provides controllable, extended release profiles up to twenty four (24) hours and, particularly, where an active drug ingredient exceeds 50% by weight of the solid preparation. Such a preparation would enable once daily dosing to fulfil, e.g., geriatric patient compliance for daily drug administration.